Summary

The present work explored the modelling as well as the analysis of the fundamental aspects related to the fluid dynamics and the problem of heat transfer between a working fluid and the matrix of a regenerative heat exchanger. Regenerators are a time dependent class of thermal energy storage systems that facilitates the transfer of heat from a hot fluid to a cold fluid through temporary retention of thermal energy in high thermal capa- city matrices. They are an attractive technology and constitute the crucial part of many thermal systems that operate with regenerative cycles, particularly the magnetic refrige- ration cycle. Firstly, a simplified model was adopted in order to investigate the behaviour of the similarity parameters that describe the heat transfer between a solid cylinder and a transversal periodic flow - the fundamental problem. The flow frequency that yields to optimum heat exchange was determined for each corresponding convective heat transfer coefficient, using a numerical correlation written in terms of the dimensionless Biot and Fourier numbers. Next, a parallel plate regenerator is studied using a mathematical model that takes into account the fluid dynamics and the coupled heat transfer between the wor- king fluid and the solid material. A hybrid solution methodology has been adopted, which encompasses analytical and numerical approaches. The reciprocating velocity field inside the regenerator was analyzed in terms of its governing similarity parameter, the kinetic Reynolds number. Parameters related to the system performance such as effectiveness, efficiency and the entropy generation number were simulated aiming to understand the regenerator thermal behaviour. The hydraulic diameter is the key factor to reduce ther- modynamic losses and the entropy generated is a useful tool to indicate situations where the system begins to present non-operant conditions. Finally, this parallel plate model is adapted to simulate an active magnetic regenerator (AMR) operating in a magnetic refrigeration cycle. Parameters such as the mass flow rate, the cycle frequency, the flow channel size were evaluated in the simulations, which provided a comprehensive picture of the AMR dynamic thermal behaviour. It was verified that the volume of fluid dis- placed inside the channels plays an important role in the quantitative assessment of the magnetic refrigerator performance. The results agree well with the trends observed in the experimental data and other theoretical models available in the literature.

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